Add information on junctions to wire documentation

A while ago, I added junctions to some wires. Today, I needed to do it again, and couldn't remember what keys to hold down while dragging. The documentation doesn't mention junctions at all.

Once I found out that they were called junctions (searching for "node red split wire" doesn't produce useful results) I found something in the 3.0 release notes but it took over 10 minutes in all.

Could something be added to the documentation covering this feature? I can submit a PR for the text, but I'm not really in a position to generate helpful images at the moment unless I just lift the ones from the release notes.

从 wire 文档页面开始，并将其与链接的 Node-RED 3.0 junctions 发布说明进行比较。记录如何创建 junctions，包括拖动时需要按住的按键；如果有可用的图片，也请添加有帮助的图片。文档应让用户无需在其他地方搜索即可发现并使用此功能。